This invention relates to the incorporation of liquid additives in the production of thermoplastic films, such as polyethylene film.
In the production of thermoplastic films by a melt extrusion process, polymeric resin, generally in the form of pellets, is advanced, typically by a screw mechanism, into a heating zone where the polymeric resin is melted and thereafter extruded through a die to form a film. Often liquid additives are incorporated in the polymeric resin to be extruded to impart various desired properties to the film. Such additives include but are not limited to cling agents, anti-static agents, anti-fogging agents, deodorants, fragrances, anti-oxidants, and the like. One way of incorporating the liquid additive into the polymeric resin is to introduce the liquid additive directly to the resin pellets before they are fed into the melt zone. With higher liquid amounts, this method often causes excessive slippage of the pellets at the screw, which deleteriously affects the feeding of the pellets to the extruder and renders difficult the process of extruding an acceptable film containing the additive. This is largely due to an uneven distribution of the liquid additive over the surface of the pellets causing excessive differences in the frictional relationships between the individual pellets, between the pellets and the extruder screw, and between the pellets and the extruder barrel. This leads to the inability to achieve the proper transfer and melt rates during the extrusion process. The result is surging of the feed and unacceptable variations in the thickness of the film, or even the shut-down of the extrusion process altogether. Thus, by simply introducing the liquid additive directly to the resin pellets of the feed, it has been generally difficult or even impossible to attain the liquid additive concentrations which are often necessary to impart the desired properties to the film.
In order to overcome the difficulties caused by introducing the liquid additive directly to the feed, it has been the practice to incorporate the liquid additive into the polymeric resin while it is in a molten form, and then to form the resin into pellets. The formation of this so called "hot-compounded" resin requires a hot compounding step which is energy intensive, requires extra process steps, and is costly.
Another prior art method for avoiding the screw slippage problem, during the incorporation of a liquid additive into a film, is to feed the liquid additive under pressure directly into the extruder barrel beyond the zone where the pellets are melted. A disadvantage of this method is that inadequate mixing occurs due to the physically incompatible materials meeting too close to the extrusion zone of the melted resin. This results in a melted extrudate which is not adequately mixed with the liquid additive. The variations in film quality that result are generally unacceptable.
Still another prior art method of incorporating a liquid additive in the polymeric resin to be extruded is to form a concentrated masterbatch of the liquid additive mixed with a pelleted polymeric resin that is compatible with the resin used to form the film. This "masterbatch concentrate" is then introduced to the resin pellets being fed into the extruder. A difficulty with this method is that the maximum amount of liquid additive that can be added to the pelletized resin of the masterbatch is often too small to provide a masterbatch concentrate having sufficient liquid additive to perform its function in the film. The low concentration of liquid additive in the masterbatch concentrate then requires an excessive quantity of masterbatch concentrate, resulting in a greater expense to handle the large amount of material of the masterbatch.
In view of the prior art problems, an object of this invention is to provide a method for introducing a liquid additive into an extruded film by the masterbatch method wherein the concentration of liquid additives in the masterbatch is at a high level not previously possible.
Other objects will become apparent from the ensuing disclosure and appended claims.